1. Field of the Invention
The present invention relates to a bearing and a pin for use in a chain and to a chain such as a silent chain and a roller chain using them.
2. Description of the Related Art
In general, a silent chain causes a relative rotational sliding motion between a pin and a link plate, and a roller chain causes a relative rotational sliding motion between a pin and a bushing, so that the pin and the fitting member (the link plate or the bushing) wear and wear elongation is caused within the chains. A silent chain such as a timing chain disposed within an internal combustion engine in particular is required to have high durability even under a condition close to a boundary lubrication state in which sliding heat is concerned.
Hitherto, Japanese Patent No. 4401108 has proposed a chain pin including a surface layer composed of vanadium carbide (VxCy) and a boundary portion composed of carbides of vanadium and chrome between a pin base material and the surface layer. In the chain pin, a chrome content in the boundary portion is changed in inclination such that the chrome content is gradually reduced from the pin base material to the surface layer.
The chain pin has the surface layer composed of the vanadium carbide having high surface durability and the boundary portion composed of the carbides of vanadium and chrome formed in a manner not forming an interface clearly divided to improve adhesion strength between the surface layer and the pin base material, to prevent separation between the surface layer and a boundary surface, and to improve durability of and to prolong a life of the chain.
With a late rise of interest in environmental and energy problems, a demand on sustainable development of an internal combustion engine and others is also increasing and a further improvement on fuel efficiency of an engine-powered vehicle is urgently required. Meanwhile, it has become an important issue how to assure long-term reliability of the timing chain described above. In such a next generation engine, there are such cases that viscosity of lubricant oil is lowered or an amount of lubricant oil is made lean due to the changes of an engine mechanism. A case where a chain lubrication environment is made severe is increasing as a lubrication condition approaches to boundary lubrication even in a case of a mixed lubrication condition. Then, it was found that there is a case that the pin having the surface layer composed of the vanadium carbide described above (referred to as a ‘VC pin’ hereinafter) causes abnormal wear in such a chain driving test.
As a result of ardent study on the abnormal wear of the VC pin described above, the inventors et al. have analyzed as follows. Firstly, a mechanism why the vanadium carbide (VC) coating has a high wear resistant performance more than that of other MCs (M: metals such as Cr, Nb, V, and Ti) type hard carbide coatings such as chrome carbide (CrC), niobium carbide (NbC), and others in the conventional chain within an engine is:
(i) Because a very thin and soft oxide film is continuously formed on the surface of the VC coating and a pin sliding surface tends to be made into a mirror surface, attackability against another member (surface of a hole of a link plate) is lowered; and
(ii) The VC coating has toughness higher than that of the other MC type carbide coatings, so that destruction of the coating (surface roughness caused by micropeeling) hardly advances even under a high contact pressure, and the mirror-surfaced sliding surface can be maintained for a long period of time.
Then, the inventors have presumed that the abnormal wear of the VC pin has occurred in the chain driving test carried out by assuming the next generation engine due to the following reasons. That is, in a circumstance in which the lubrication environment is severe, a lubrication condition of the sliding portions (the surface of the pin and the surface of the hole of the link plate) approaches to the boundary lubrication condition, and the pin surface generates heat, becoming hot. As a result, the oxide film accelerating the drop of the attackability against the other member is formed thickly and the wear of the pin itself is increased because the soft oxide film is worn.
It is assumed that the demand on the chain will become severe more and more with the progress of the internal combustion engine such as further cut of fuel consumption of vehicles. A chain having durability enabling to drive for a long period of time is required even under such circumstances that not only the viscosity of the lubricant oil is lowered but also the lubrication environment is aggravated due to an increase of a chain load tension and others.